1. Field of the Invention
The present invention relates to an outboard motor.
2. Description of the Related Art
In recent years, outboard motors equipped with catalytic converters have been introduced to improve environmental performance. When a catalytic converter is installed in an outboard motor, it is necessary to provide a cooling structure to cool the catalytic converter. It is also necessary for the catalytic converter to be warmed to a prescribed temperature or higher to activate the catalyst. Therefore, in the outboard motor exhaust apparatus disclosed in Laid-open Japanese Patent Application 2008-169707, a thermostat is arranged in a cooling water flow passage downstream of the catalytic converter. The thermostat opens the cooling water flow passage when a temperature of the cooling water reaches or exceeds a prescribed temperature. The thermostat closes the cooling water passage when the temperature of the cooling water is below the prescribed temperature. Thus, the cooling water is stagnant until the catalytic converter reaches the prescribed temperature and, as a result, early activation of the catalyst is promoted.
Additionally, in the outboard motor exhaust apparatus disclosed in Laid-open Japanese Patent Application 2008-169707, a thermostat is arranged in the cooling water flow passage at a position downstream of the cylinder block. As a result, it is possible to warm up the cylinder block. Thus, when the cooling water passage is branched into two systems, the warming state of each of the systems can be controlled individually by providing a thermostat in each of the systems and opening and closing the thermostats according to the temperatures of the respective systems.
When the cooling water passage is branched into a plurality of systems, the total cross sectional area of the cooling water passage increases. Consequently, a larger water pump is necessary to feed the cooling water. A larger water pump is a problem because it requires a larger bottom casing to house the water pump.
Conversely, if the cooling water passage is not branched, then the total cross sectional area of the cooling water passage is not so large and a larger water pump can be avoided. However, when the cooling water passage is not branched, it is necessary to arrange a thermostat downstream of both the catalytic converter and the engine in order to warm up the catalytic converter and the engine. In such a case, the passage length of the cooling water passage from the water pump to the thermostat is long and it is easy for an uneven temperature distribution to occur along the cooling water passage. For example, when the engine is operated with a fully open throttle after starting, a situation occurs in which the cooling water temperature is high near the catalytic converter but low near the thermostat. In such a case, the thermostat will remain closed and the cooling water will be stagnant even though the catalytic converter is at a high temperature. Consequently, it is difficult to sufficiently cool the catalytic converter.